Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration. Measurements made in a borehole (i.e., down hole measurements) are typically performed to attain this understanding, to identify the composition and distribution of material that surrounds the measurement device down hole.
For example, Laterolog and induction resistivity tools can be used to measure the resistivity profile of formations along boreholes to evaluate petrophysical parameters. Following their introduction in 1910-1950's, these tools have evolved and grown more sophisticated, to include the use of sensor arrays. Array tools can produce a radial profile of resistivity, enabling the measurement and correction of borehole and invasion effects. Focusing methodologies that improve resolution vertically and radially can also be implemented.
However, due to the varying distances and focusing mechanisms employed by array tools, the strength of signals at different receivers can differ by orders of magnitude. This leads to special design and operational requirements, including a wide dynamic range. One consideration related to dynamic range is the interference that occurs between different modes (transmitters, components, frequencies) of the tool. Since the level of interference is proportional to the separation between different modes in time or frequency, there is a trade-off between having low-interference and a reduced listening time, which is most noticeable when using low-frequency instruments (e.g., those attached to a Laterolog tool).